1. Field of the Invention
The present invention relates to a method of manufacturing a nitride semiconductor and, more particularly, to a method of manufacturing a non-polar m-plane nitride semiconductor.
2. Description of the Related Art
In general, a group III nitride semiconductor is characterized by capability of emitting light in a wide range including the entire visible ray region and extended to the ultra-violet ray region, and has gained attention as a light emitting device material for realizing blue or green.
A nitride semiconductor is grown on a composite substrate such as sapphire (Al2O3) or silicon carbide (SiC) through vapor phase growth methods like Metal Organic Chemical Vapor Deposition (MOCVD), Hydride Vapor Phase Epitaxy (HVPE) or Molecular Beam Epitaxy (MBE).
Conventionally, a nitride single crystal grown in a c-axis [0001] of the composite substrate has been mainly used for the nitride light emitting device, which however, exhibits strong piezoelectricity, degrading the efficiency of the light emitting device. That is, in the active layer of the c-plane nitride layer, an electric field stemming from the piezoelectric effects, i.e., a piezoelectric field is applied in an opposite direction from an external electric field due to the peculiar characteristics of GaN. As shown in FIG. 1a, the wave functions of an electron and a hole are spatially in disagreement, resulting in degraded recombination efficiency.
Recently, in order to increase the light emission efficiency by controlling such polarization characteristics, there have been actively sought ways to grow and apply an a-plane or m-plane non-polar nitride layer to the light emitting device. Unlike the polar nitride layer, the non-polar nitride layer does not entail polarization, and thus no electric field is applied internally. Therefore, in the active layer with the non-polar nitride layer adopted, the wave functions of the electron and hole almost ideally coincide with each other as shown in FIG. 1b. 
As a conventional method of growing a non-polar nitride layer, U.S. Patent No. 2003/0198837 (published on Oct. 23, 2003 and invented by Michael D. Craven et al.) suggests using an r-plane (10-12) sapphire substrate or an a-plane hexagonal substrate such as ZnO or SiC to grow an a-plane (10-10) nitride layer.
However, not as stable as the m-plane nitride layer in terms of surface energy, it is known that stripe patterns are formed on the surface of the a-plane nitride layer during growth. Additionally, as in FIG. 2, macro pits are formed on the surface due to the anisotropy in growth rate along with c-axis. Such pits provide paths for leakage current in the light emitting device, thus causing significant deterioration of the light emission efficiency.
On the other hand, the m-plane (10-10) nitride semiconductor is non-polar like the a-plane, but has lower surface energy and thus is more stable, thereby resulting in a relatively planar surface during the growth. Such an m-plane nitride semiconductor has a unique lattice constant, so the substrates such as LiAlO2, m-plane ZnO and m-plane SiC have been used experimentally as a growth substrate.
However, these substrates have demerits as a growth substrate for the m-plane nitride semiconductor. For example, the LiAlO2 substrate is too expensive, the ZnO substrate is chemically unstable, and the SiC substrate is limited in area like the ZnO substrate, hindering the growth of the m-plane semiconductor.